A digitally controlled impedance (DCI) calibration scheme can be used to compensate for changes in pull-up and pull-down impedances in an output driver (hereinafter “driver”). The driver pull-up and pull-down impedances are calibrated against a precision reference resistor, which is located external to the integrated circuit (IC) die (e.g., on a printed circuit board (PCB) to which the IC die is attached). The driver pull-up and pull-down impedances are calibrated to compensate for process variation, as well as for temperature and supply voltage fluctuations (generally process, voltage and temperature (PVT) variations). The external precision reference resistor can have a small resistance variation from the nominal value. The driver can be designed to ensure that the range in drive strength covers the entire range of the variation in resistance of the external precision reference resistor.
In some cases, it is desirable to use an on-die reference resistor with a DCI calibration scheme. For example, for a High Bandwidth Memory (HBM) interface, where the memory controller and stack dynamic random access memory (DRAM) IC are placed beside each other in the same package. The interface between memory controller and DRAM is through silicon interposer, not through the package and PCB. To save package pin and PCB board real estate, the on-die reference resistor is desired.] An on-die reference resistor has a larger variation in resistance from the nominal value as compared with an external reference resistor due to process variation. That is, the difference between the fast resistor process corner and the slow resistor process corner for an on-die reference resistor is larger than the variation in resistance of an external reference resistor. Designing a driver to have a range in drive strength that covers the entire range of an on-die reference resistor consumes more power and die area as compared to an external reference resistor. It is desirable to provide a driver and associated DCI calibration scheme that consumes less power and less die area when using an on-die reference resistor.